1. Field of the Invention
The present invention relates to an area recognizing device for recognizing an edge area and a non-edge area of an image which is expressed by an N-level gradation level, and to a gradation level converting device employing the area recognizing device (N&lt;M).
2. Description of the Related Art
Recently, outputting devices, such as a printing device and a display device, print and display multi-level data. When image information is processed electronically, an information amount is inevitably increased.
In order to tackle the problem of increased information amount, a facsimile apparatus, a still image file, and a copy device convert an original image into binary signals, then transmit, store, and process the binary signals. However, image quality is degraded if the binary signals are directly reproduced with the outputting devices.
To improve image quality, the binary signals are returned into multi-level gradation signals in reproducing with the outputting devices. Consequently, the degradation of image quality is avoided.
A dither method and an error-diffusion method are employed to express the intensity of color by binary signals, and the binary signals of this digital halftone image are returned into multi-level signals by a known halftone image estimating method. For example, "METHOD AND APPARATUS FOR ESTIMATING HALFTONE IMAGE FROM BINARY IMAGE" cited in U.S. Pat. No. 4,758,897 is a well known method. With this method, a halftone image is estimated based on a ratio of black and white pixels within a scanning aperture to be scanned. More specifically, a larger scanning aperture is selected in an area where the fluctuation in pixel level is small, while a smaller scanning aperture is selected in an area where the fluctuation in pixel level is large. Then, a halftone image is estimated based on a ratio of black and white pixels within the thus selected scanning aperture.
Also, the binary signals are converted into the multi-level signals as eliminating any influence of patterns of the original image being different depending on cycles by detecting edges with a plurality of differentiators, and selecting the detected edge signal (for example, Japanese Laid-Open Patent Application No. 3-250975).
Also, edges of the binary-level image are detected by a single edge-detection filter, then an edge area, a non-edge area, and an intermediate area are recognized based on outputs from the edge-detection filter. The degree of smoothness of the image is adjusted by changing weighing coefficients to be added within the scanning aperture (for example, Japanese Laid-Open Patent Application No.4-51378).
However, desired uniformity and resolution are not obtained simultaneously by simply controlling the aperture size and weighing coefficients of a filter which estimates a halftone image based on a ratio of black and white pixels within the scanning aperture of the binary image.
Also, to implement the method of detecting edges of the binary image with a plurality of edge filters, and selecting a single edge filter based on the detected result, the process of switching between an edge area and a non-edge is needed. Consequently, negative effects of detection errors appear in a display image. With a single edge detector, on the other hand, the detection is influenced by the texture of a dot arrangement pattern in converting into the binary signals; as a result, recognition errors frequently occur.